Selenium is an essential micronutrient in the diet of humans and other mammals. Many health benefits have been attributed to selenium that include preventing various forms of cancer (e.g., colon, prostate, lung and liver cancers), heart disease and other cardiovascular and muscle disorders. Numerous human clinical trials have been undertaken in recent years to assess the role of this element in cancer prevention, delaying the progression of AIDS, etc., at a cost of hundreds of millions of dollars, but little was known about the mechanism of how selenium acts at the metabolic level in mammals to incorporate selenium into protein. We proposed several years ago that the health benefits of selenium are due largely to the presence of this element in selenoproteins as the selenium-containing amino acid, Sec. In the ensuing years, we established the biosynthetic pathway of Sec in eukaryotes and Archaea and focused on the two Sec tRNA isoforms that we demonstrated were responsible for the synthesis of the two subclasses of selenoproteins, housekeeping and stress-related selenoproteins; and pursued studying the methylase, designated Um34 methylase, that synthesizes the methyl group at the 2'-O-postion on the ribosyl moiety at nucleotide 34 of Sec tRNA. We provided strong evidence that addition of Um34 to the isoform, 5-methylcarobxymethyl-uridine (mcmU), to form 5-methylcarboxymethyl-2'-O-methyluridine (mcmUm) requires that mcmU is aminoacylated with Sec, i.e., that the substrate for the methylase (designated Um34 methylase) which carries out this reaction is selenocysteyl-tRNA. In addition, our program focused on developing mouse models to assess the role of all selenium-containing proteins within the two subclasses, housekeeping and stress-related selenoproteins, and on individual selenoproteins in preventing and promoting cancer and in mammalian development. In the past year, we have completed and published the following projects: 1) analysis of the first selenocysteine (Sec) tRNA mutation found in a family, wherein the parents were heterozygous for a G-C base change at position 65 and the proband was homozygous. The patient, an eight year old male, suffers from abdominal pain, muscle weakness and fatigue. The level of mutant Sec tRNA was reduced substantially compared to normal tRNA and the proband synthesized stress-related selenoproteins poorly; 2) dietary selenium regulates the Sec content in selenoprotein P, the only selenium-containing protein in mammals that contains multiple Sec residues, and the replacement of Sec by cysteine; 3) loss of the glutathione peroxidase 4, a selenoprotein, was far more detrimental than the loss of the entire selenoprotein population in mouse liver, and vitamin E could compensate for its loss; 4) an examination of the antioxidant proteins in human liver and lung tumors and comparing them to antioxidant proteins in the corresponding surrounding normal tissues and to each other revealed a greater dependence of liver on either the thioredoxin or glutathione system to drive the malignancy, while lung cancer appeared to depend primarily on the thioredoxin system; 5) an analysis of the role of selenophosphate synthetase 1, an essential protein, in regulating redox homeostasis in mammals using a conditional knockout mouse model; 6) the observation that the loss of thioredoxin reductase 1 or the 15kDa selenoprotein (Sep15) in a mouse colon cancer cell line reversed several malignant properties of the cancerous properties, but surprisingly the simultaneous loss of both of these antioxidant selenoproteins reversed the anticancer effects; 7) knockout of SECp43 (function unknown) in mouse liver was found to be embryonic lethal, but had no apparent role in selenoprotein expression in liver even though it was known to form a complex with Sec synthase and Sec tRNA; 8) observation that the effects of dietary selenium levels on immune function are not limited to liver and further identify the IL-6 and interferon-gamma pathways as being responsive to dietary selenium intake; and 9) two studies involving loss of Sep15 in Chang liver cells inhibiting cell proliferation and motility at the G1 phase and cytoskeleton remodeling and membrane blebbing. In addition, a manuscript on an analysis of extensive ribosomal frameshifting in Euplotes is under revision at Nature Struct. Biol. and the 4th edition of our book entitled Selenium: Its Molecular Biology and Role in Human Health, containing 50 chapters, was completed and will be published in 2016.